The electrostatic application of coating materials such as paint is most often carried out by applying a high voltage charge to a powder or atomized mist of liquid coating material which is sprayed into the air in the vicinity of the objects to be coated, and then electrostatically attracting the airborne charged material toward the object, which is grounded or charged at the opposite potential. The charged material so attracted envelopes and thereby coats the entire object.
In a number of coating applications, the use of water-based liquid coatings has found increasing desirability. For example, the use of water-based paints produces less solvent emissions, which must be carefully controlled in a plant, than the use of organic-solvent based paints. In addition, many flammable solvent-based paints present fire hazards, whereas water-based paints are less flammable. Furthermore, the cost of solvent-based paints has become much higher than that of water-based products. These considerations, as well as the availability of improved water-based coatings, have motivated a shift toward the water-based coatings. This shift has been significant where spraying paint onto large objects on open assembly lines is employed, as, for example, with the painting of cars in the automobile industry.
The use of water-based paints has its own problems. A problem presented by the use of water-based paints in electrostatic spray systems is that the water-based coating liquids are conductive. For this reason, the fluid path from the paint supply reservoir to the spray orifice, where the charging electrode is typically located is a continuous liquid conductor. As such, the use of water-based paints with electrostatic spray devices requires electrical isolation of the entire paint supply system. Otherwise, the charging electrode of the gun will be grounded out through the paint supply and the entire system would present a shock hazard to operating personnel.
In large scale manufacturing facilities, such as those found in the automobile industry, electrical isolation of the paint supply system is indeed a monumental task. Such systems involve large paint tanks and extensive piping throughout large automotive plants which makes effective electrical isolation of the system highly impractical.
Attempts to charge an atomized spray of water-based coating material after it is discharged from the spray device so that the supply system may itself be kept at ground potential have been attempted by using inductive or other methods. Such methods of the prior art have not adequately focused the electrostatic field on the product being painted to control overspray.
Accordingly, there has been a need for an effective method and apparatus by which a water-based or other conductive spray coating can be charged and electrostatically attracted in a focused way to an object to be coated. There has been a particular need for an effective method and apparatus for electrostatically applying water-based or other conductive coating material in a coating system where isolation of the coating supply is impractical or undesirable.